An optical or radio-frequency (RF) source is commonly referred to as a frequency comb when it is represented by multiple tones that are equidistantly spaced in the frequency domain. Optical frequency combs are one of the key elements in Fourier analysis, which, in turn is fundamental to the operation and analysis of linear systems. Explicitly, an optical frequency comb is a set of lines equally spaced (in this context) in frequency. In the engineering practice, optical frequency combs are used in metrology (taking advantage of their uniform pitch) whereas in optics they represent an indispensable tool for signal processing. Additionally, optical frequency combs may be of benefit in wavelength division multiplex (WDM) transmission systems which are organized as multi-channel systems, each channel occupying a pre-assigned frequency bin (often of 50 GHz), where they can serve either as carriers of information, or local oscillators at the receiving end of an optical link, used for the demodulation and retrieval of information. Thus, the ability to align, or lock, optical frequency combs mutually in frequency is critical in a number of disciplines.